The selective etching of polyimide films to provide openings or vias therein is important for various uses of polyimide. For instance, in the packaging of semiconductor chips, substrates are often provided with polyimide layers.
In a number of these situations, it is necessary to form vias in the polyimide layer to allow for electrical connections to be made between the different layers of metallurgy. In order that the interconnection be as accurate as possible, it is necessary that the polyimide films be fully cured to avoid distortion of the desired polyimide pattern and prevent attack from other wet processing chemicals.
For instance, in the formation of multi-layer substrates for mounting chips, one configuration employs an insulating substrate of ceramic material onto which is deposited a pattern of metallic conductors. Usually, the conductors are three layers of metal being a layer of chromium or nickel, followed by a layer of copper, followed by an overlayer of chromium or nickel. On top of the metallized ceramic substrate is placed a film or layer of polyimide; on top of the polyimide a second layer of pattern conductors is provided. The conductors are generally three layers of metal being chromium or nickel, followed by copper, followed by chromium or nickel. It is necessary to electrically contact some of the conductors in the upper or second layer of metallization to some of the conductors on the lower or first layer of metallization. In order to do so, the polyimide must be selectively etched to form the desired vias therein to allow for metal connection between the upper and lower levels of metallization.
Another use would be as a dielectric and/or circuit carrier for flexible circuits. This would involve, for example, spray coating or roller coating polyamic acid onto a sheet of metal (such as stainless steel or aluminum). The film is then thermally cured or imidized, resulting in a film which is fully or substantially fully thermally cured. The metal which the polyimide is on can be imaged, removed, or maintained. On top of the polyimide, three layers of metal are deposited such as by either evaporation or sputtering. The conductors are chromium or nickel, followed by a layer of copper, followed by a layer of chromium or nickel. By means of photolithographic operations, this metal is imaged into circuits. Depending on the use of the circuit, the polyimide may or may not be imaged either before or after the formation of the circuit.
One technique for wet etching fully or substantially fully cured polyimide is using either hydrazine hydrate or ethylene diamine. Both are extremely dangerous chemicals and are avoided by industry wherever possible. Ethylene diamine is highly carcinogenic and hydrazine hydrate is highly unstable and extremely explosive. If used, both would have tools which are extremely expensive because of safety concerns.
Moderate or slightly thermally cured polyimide (significant amounts of polyamic acid remain) can be etched by using a metallic hydroxide. Etch time is dependent on etchant temperature and concentration, the films degree of cure (percent imidization), and film thickness.
In addition, chemically cured polyimide can be etched using relatively high concentrated hot potassium hydroxide such as about 8 to about 15 molar.
Furthermore, the by-products formed by the etching are somewhat difficult to remove.
Accordingly, methods to etch polyimides and especially fully or at least substantially fully cured polyimides have not been entirely satisfactory and could stand significant improvement.